Effect of Carrier Doping on Nonlinear Distortion of Microwave Signals by Superconducting Thin Films
Dr. Stephen Remillard
The performance of high temperature superconductors is limited by signal distortion under the influence of high electric current. Measurements of nonlinear distortion made on Tl2Ba2CaCu2O8-x (TBCCO-2212) wafers using a 5.5 GHz sapphire dielectric resonator reveal dependence on carrier doping x, with grain boundaries contributing more significantly in underdoped films (x>0.1) as compared to those optimally doped (x=.1). The doping level was tuned by annealing the TBCCO- 2212 thin films in a reducing nitrogen atmosphere at temperatures ranging from 250oC to 400oC. The microwave current dependent surface impedance of both pre- and post-annealed films was measured. With the critical temperature being used as the indicator of carrier density, it was found that underdoped samples have larger nonlinear grain boundary losses as indicated by a weaker variation of surface reactance with surface resistance, revealing the preparation conditions for optimal wafer performance.
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